14-11-2012, 01:59 PM
Dynamic Load and Stress Analysis of a Crankshaft
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ABSTRACT
In this study a dynamic simulation was conducted on a
crankshaft from a single cylinder four stroke engine.
Finite element analysis was performed to obtain the
variation of stress magnitude at critical locations. The
pressure-volume diagram was used to calculate the load
boundary condition in dynamic simulation model, and
other simulation inputs were taken from the engine
specification chart. The dynamic analysis was done
analytically and was verified by simulation in ADAMS
which resulted in the load spectrum applied to crank pin
bearing. This load was applied to the FE model in
ABAQUS, and boundary conditions were applied
according to the engine mounting conditions. The
analysis was done for different engine speeds and as a
result critical engine speed and critical region on the
crankshaft were obtained. Stress variation over the
engine cycle and the effect of torsional load in the
analysis were investigated. Results from FE analysis
were verified by strain gages attached to several
locations on the crankshaft. Results achieved from
aforementioned analysis can be used in fatigue life
calculation and optimization of this component.
INTRODUCTION
Crankshaft is a large component with a complex
geometry in the engine, which converts the reciprocating
displacement of the piston to a rotary motion with a four
link mechanism. This study was conducted on a single
cylinder four stroke cycle engine.
Rotation output of an engine is a practical and applicable
input to other devices since the linear displacement of
an engine is not a smooth output as the displacement is
caused by the combustion of gas in the combustion
chamber. A crankshaft changes these sudden
displacements to a smooth rotary output which is the
input to many devices such as generators, pumps,
compressors.
LOAD ANALYSIS
The crankshaft investigated in this study is shown in
Figure 1 and belongs to an engine with the configuration
shown in Table 1 and piston pressure versus crankshaft
angle shown in Figure 2. Although the pressure plot
changes for different engine speeds, the maximum
pressure which is much of our concern does not change
and the same graph could be used for different speeds
[9]. The geometries of the crankshaft and connecting rod
from the same engine were measured with the accuracy
of 0.0025 mm (0.0001 in) and were drawn in the I-DEAS
software, which provided the solid properties of the
connecting rod such as moment of inertia and center of
gravity (CG). These data were used in ADAMS software
to simulate the slider-crank mechanism. The dynamic
analysis resulted in angular velocity and angular
acceleration of the connecting rod and forces between
the crankshaft and the connecting rod.
FE MODELING OF THE CRANKSHAFT
The FE model of the crankshaft geometry has about 105
quadratic tetrahedral elements, with the global element
length of 5.08 mm and local element length of 0.762 mm
at the fillets where the stresses are higher due to stress
concentrations. As a crankshaft is designed for very long
life, stresses must be in the linear elastic range of the
material. Therefore, all carried analysis are based on the
linear properties of the crankshaft material. The meshed
crankshaft with 122,441 elements is shown in Figure 6.
The dynamic loading of the crankshaft is complicated
because the magnitude and direction of the load
changes during a cycle. There are two ways to find the
stresses in dynamic loading. One method is running the
FE model as many times as possible with the direction
and magnitude of the dynamic force. An alternative and
simpler way of obtaining stress components is
superposition of static loading. The main idea of
superposition is finding the basic loading positions, then
applying unit load on each position according to dynamic
loading of the crankshaft, and scaling and combining the
stresses from each unit load. In this study both methods
were used with 13 points over 720 degrees of crankshaft
angle. The results from 6 different locations on the
crankshaft showed identical stress components from the
two methods.
CONCLUSIONS
The following conclusions could be drawn from this
study:
1. Dynamic loading analysis of the crankshaft results in
more realistic stresses whereas static analysis
provides an overestimate results. Accurate stresses
are critical input to fatigue analysis and optimization
of the crankshaft.
2. There are two different load sources in an engine;
inertia and combustion. These two load source
cause both bending and torsional load on the
crankshaft.
3. The maximum load occurs at the crank angle of 355
degrees for this specific engine. At this angle only
bending load is applied to the crankshaft.
4. Considering torsional load in the overall dynamic
loading conditions has no effect on von Mises stress
at the critically stressed location. The effect of
torsion on the stress range is also relatively small at
other locations undergoing torsional load. Therefore,
the crankshaft analysis could be simplified to
applying only bending load.